Running adverts on third-party sites through the Google Display Network is the next best thing after search advertising, in my experience. I’ve rarely come across an individual site where direct advertising with that site alone produces better value results than the Display Network. Why book with one (presumably good, but expensive) site when you can book with hundreds or even thousands of sites and almost inevitably find better ones at a lower cost?
However, managing ads on the Display Network requires a lot of care. There are all sorts of sites where you wouldn’t want your advert to appear, which is why at BMON we usually build up a list of approved sites for clients from scratch, rather than target all sites but with an ‘exclusion list’ of poor or inappropriate ones.
When we do take the latter approach, we have a list of thousands of low quality sites which we can exclude. I imagine most agencies have the same. Now you (and indeed we) can draw on the experience of many agencies with a tool called ExclusionSites. It’s a great idea, and there’s an introductory article about it on Search Engine Land here.